Vesicles of the Utricularifc. 397 



and exhalation take place through the whole surface of the epi- 

 bleina. Experiments made by MM. Cloez and Gratiolet have 

 demonstrated that the decom])osition of carbonic acid by the 

 p,reen parts of submerged plants is only effected under the in- 

 tiuence of ligdit. In the dark no carbonic acid is produced, con- 

 trary to what occurs in aerial plants. A certain temperature is 

 necessary for the production of the phenomenon. When the 

 temperature is rising, it does not commence below 15° C. 

 ( = 59° r.); when the temperature is descending, it may con- 

 tinue at 10° C. ( = 50°F.). Besides oxygen, the gas produced 

 by the plant contains a certain rpiantity of nitrogen. 



If we apply the preceding facts to the leaves of the Utricu- 

 larice, we find them immersed in a water which is usually very 

 rich in carbonic acid ; this gas is absorbed by the leaves, and, 

 under the influence of light, oxygen and a little nitrogen are 

 evolved. In the sccments of the leaves the cases occur in the 

 aeriferous canals which traverse those segments; they are set 

 free at different points in the form of little bubbles. We have 

 not seen these bubbles issuing through the walls of the utricles, 

 which appear to oppose a certain amount of resistance to their 

 escape, and this may probably exert some action upon their own 

 expansion. The utricles floating freely in the water become the 

 seat of phenomena of endosniose and of chemical actions. This 

 W'Ork goes on especially when the circumambient water presents 

 a temperature of 10°-15° C. ( = 50^-59° F.). At first the utri- 

 cles contain a mucilaginous liquid ; soon a bubble of gas is seen 

 to make its appearance in this liquid, and to increase in volume: 

 this is oxygen, evolved under the influence of light and heat 

 which have penetrated through the water to the utricles. The 

 plant escapes from the mud and rises towards the surface; the 

 secretion of gas becomes more abundant, and the flower-stalk is 

 supported above the level of the water. The oxygen secreted in 

 the utricles seems to exert a cheaiical action, in virtue of which 

 the contents of the cells of the walls undergo a transformation 

 and a change of colour, becoming rose, lilac, and blue. The 

 envelope thus coloured reacts in its turn upon the interior work 

 of the utricles. We know that, in fact, in organs coloured 

 otherwise than green (for example, in the petals of the corolla), 

 there is no longer any evolution of oxygen, but, on the contrary, 

 absorption of oxygen and evolution of carbonic acid ; the latter 

 does not escape from the utricle, but is probably assimilated; 

 the utricle becomes filled again either with a mucilaginous mat- 

 ter or with water absorbed by endosmose, and the plant increases 

 in weight and descends again to the bottom of the watcr-l^. 



* In the dark, for example in the utricles which are of a black-blue 

 colour, carbonic acid is no longer formed. 



